1. Field of the Invention
This invention relates to a polarization conversion method and related devices for a polarized light source, and in particular, it relates to such light sources useful in projection systems.
2. Description of the Related Art
A typical projection system includes a light source device, the light source device including multiple monochromatic light sources, such as and not limited to red, green and blue light sources, to generate red (R), green (G) and blue (B) lights, respectively.
To reduce cost and increase brightness of the output light, conventional light source devices generate multiple monochromatic lights by employing light emitting elements and a wavelength conversion device having wavelength conversion materials. The wavelength conversion materials include phosphor powders, dyes of mano light emitting materials. An example of a wavelength conversion device is a segmented wheel, each segment carrying one or more phosphors. The excitation light from the light emitting element illuminates the rotating wheel to generate converted light (also referred to as exited light) having multiple alternating colors which corresponds to the multiple segments of the wheel carrying different wavelength conversion materials. For example, three-color projection systems often use an excitation light from a blue LED (light emitting diode) to generate desired red, green and blue monochromatic lights.
As is well known, when phosphor materials are excited and generates converted light, the converted light is emitted in all directions substantially uniformly. Conventional light source devices, such as UHP lamps, LEDs, and the wavelength conversion type of light source devices described above all output non-polarized light.
In conventional LCD (liquid crystal display) or LCoS (liquid crystal on silicon) projection systems, because the liquid crystal or the LCoS chip can only use polarized light of a desired polarization for display, only about 50% of the light energy is used by the projection systems. In such display systems, to increase light utilization efficiency, it is desired to convert the non-polarized light from the light source devices into polarized light. Conventional conversion technology typically uses a method shown in FIG. 1:
In FIG. 1, the function of the polarization beam splitter plate 1 is to separate the input non-polarized light beam 41 from a typical light source, by transmission and reflection, into two polarized light beams, namely P light 42 and S light 43. The polarization directions of these two polarized lights are perpendicular to each other. Then, a reflector 2 reflects the S light 43 to a half-wave plate 3, which rotates the polarization direction of the S light 43 by 90 degrees to generate P light 44. Thus, the non-polarized light from the source is converted into polarized light including P light 42 and P light 44.
However, a problem of the above technology is that the size (area) of the light beam is expanded in the polarization conversion process. Although theoretically the energy of the useable polarized light is increased by 2 with this conversion method, the etendue of the output light is also increase by 2. Thus, for the projection system, the brightness (or luminance, defined as the light flux per unit solid angle per unit area) is not increased. In fact, due to losses in the polarization conversion process, the brightness is actually decreased. Because UHP lamps have relatively low etendue, the increase of etendue due to the above polarization conversion and the brightness reduction may not be obvious; but for LED light sources which have relatively large etendue, the impact on the brightness of the projection system can be a problem.